The invention itself relates generally to steam generators and more particularly to such steam generators using a plurality of energy sources to generate the steam therefrom.
The steam generators designed to date have used a single fluid stream from a process as the energy source for the steam generation. In the case of multiple energy sources, separate steam generators, either in parallel or series, have been used to generate the steam. Such use of separate steam generators result in more individual components and complicated controls, which are consequently more complex and have more opportunities for flow induced instabilities.
In many process systems, multiple fluid streams are often used. These streams contain excessive energy which may be recovered. An attractive way to accomplish this recovery is to generate steam for use in the process system or for other independent uses (i.e., power generation, space heating, etc.).
For example, in the pulp and paper industry, huge equipment installations necessitate the use of extremely large amounts of energy. Thus, multiple fluid streams are required to produce tons of pulp and paper on a daily basis. Equipment of this nature must also be capable of operating continuously with very little down time.
Another example involves systems such as thermo-mechanical pulping (TMP) and chemi-mechanical pulping (CMP), both of which require extremely large quantities of electrical energy to operate motors (typically these motors have power ratings in the thousands of horsepower range). These TMP/CMP motors grind wood chips to a fine pulp at an output rate of more than 100 tons per day. As a result, extremely large quantities of waste energy are generated during the conversion of the electrical power to drive the motors. Subsequently, this mechanical energy is converted to thermal energy in the form of steam. This steam is then passed through a tube containing the wood chips, prior to their entry into a primary refiner, to soften the chips and to facilitate in the refining operation. However, the heat generated during the actual refining operation, which is performed in a confined region, is vented to the atmosphere through an exhaust conduit. Although some efforts have been developed to recycle small portions of the vented exhaust to the aforementioned steaming tube in which the chips are initially heated and softened, the vast majority of the heat energy is still unrecoverable and hence is lost.
Yet another example involves the processing of hydrocarbon-based fuels into a hydrogen rich gas for industrial uses, and particularly for use in fuel cells. In this example, steam is produced using the energy from multiple process streams (i.e., hot exhaust gas from the fuel cell stack). The invention disclosed below was originally developed for this particular application, although it is expected that it may be applied to any of the fields discussed herein, as well as all others known to those skilled in the art.
Attempts have been made to recover the wasted energy from the processes discussed above, as well as others. As an example, U.S. Pat. No. 4,437,316 teaches method and apparatus for waste energy recovery through the use of a working fluid. This working fluid derives heat energy from the waste energy typically exhausted from a facility upon completion of a manufacturing process step. The energy of the working fluid is then utilized to develop steam at a temperature and pressure which make the steam extremely advantageous for use in the process. Waste energy from two different locations in the mechanical process apparatus is utilized to increase the energy of the working fluid through the use of separate independent evaporators. The working fluid passes from one evaporator to a first compressor and then to a cooling tank, for de-superheating the working fluid. The working fluid from the second evaporator passes directly to a second compressor. The working fluid from the output of the condenser passes to the cooling tank. The cooling tank de-superheats the working fluid entering the cooling device and automatically adjusts the level control between the liquid/vapor phases therein, enabling the first and second compressors, which operate under control of a common prime mover, to operate at significant different temperature and pressure levels and to accommodate different mass flow rates of the working fluid.
Although the aforementioned patent demonstrates that wasted energy from two independent process sources may be utilized in the process operation, a simplified multi-stream energy source steam generator which can accommodate a plurality of independent energy sources to power a simple steam generator using a common water volume for use in numerous applications would be welcome by the industry. Likewise, a simplified multi-stream energy source steam generator which does not require use of an intermediate working fluid is desirable. A direct steam generator with a lower overall system entropy is also needed. Finally, a multi-stream energy source steam generator which does not rely upon compressors would be welcome.
The present invention solves the problems associated with prior art devices (mentioned above) as well as others by providing a single steam-generating unit heated through a plurality of independent heated surfaces for steam generation, such as independent heat exchange tubes, all in contact with a common water source or fluid stream. The invention may have particular usefulness for providing steam to a fuel cell system and/or a fuel processor system which produces hydrogen rich gas for industrial purposes (including providing hydrogen rich gas to fuel cells).
The present invention provides a useful heat exchanger to recover energy from processes that have multiple points where energy is generated and needs to be removed or recovered, thereby improving the overall efficiency of the process and permitting an increased ability to do useful work. The system provides the generation of steam in a single volume of fluid (or a single fluid stream) and generally results in improved heat transfer.
As a result of the recirculation of the fluid in this invention, the steam never becomes high quality, which forces the heat transfer into transition and further causes film boiling heat transfer regimes on the boiling side. As a result, the materials and the overall weight of the steam generator is reduced by the present invention.
The probability for two-phase corrosion and erosion of the system surfaces is also reduced, and the overall system pressure drop will usually be less. Depending on the geometry chosen, the water inventory will be increased, which provides a buffering effect for transients and allows more time for controller action.
Since only a single multi-stream steam generator is needed according to this invention (rather than separate generators), the controls are simplified, but these controls still permit the energy of the individual heating streams to be removed.
Due to the characteristics of the fluids in each stream, it may be desirable to keep the heating fluids isolated because of their potential physical interaction. However, isolation of the heating fluids also has the added benefit of requiring two separate boundary failures before the heating fluids from two steam generating areas would be mixed. Ultimately, this isolated fluid set up results in a more reliable separation of the fluids than previously thought possible.
In summary, the present system requires less material, is easier to operate and control, is more efficient and reliable, and is safer for use in many processes where mixing may cause an accident.
Accordingly, the present invention comprises a steam generator having a plurality of energy sources for heating a common water inventory to generate steam. The generator consists of a steam generating vessel with a feed water inlet and a steam outlet and a plurality of heat exchangers located in the steam generating vessel with each individual heat exchanger being supplied with heat energy from different fluid sources.
Notably, the present invention does not raise the potential energy of the fluid stream by performing work on it. Instead, the essence of the present invention focuses on transferring energy already present within the system to conserve energy and/or perform additional work upon the system, thereby making the entire system more efficient.